1. Field of the Invention
The present invention relates to correlating optical and virtual images in electron microscopy.
2. Description of Related Art
A Scanning Electron Microscope (SEM) operates by rastering (scanning) a sharply focused beam of electrons over the surface of a specimen and then displaying on a separate display device a virtual image corresponding to the changing pattern of response signals emanating from the specimen in response to interaction between the electron beam and the specimen. Thus, a point on the specimen which produces a weak response signal appears as a corresponding dark point on the display device, and a point on the specimen which produces an intense response is recorded as a correspondingly bright point on the display. A magnified image is achieved by scanning the electron beam over a small region of the sample and then displaying the response as a virtual image on a much larger display surface. By using a very small electron probe and scanning over very tiny areas, it is possible to achieve magnifications of tens of thousands of times and resolve features in the submicron and even nanometer scale ranges. However, on the other end of the SEM magnification range, because of various practical considerations, a SEM cannot generally scan its beam over an area greater than approximately 1 cm square. In order to inspect larger specimens, it is necessary to manipulate a mechanical positioning stage to bring the desired region of the specimen into view and in this context the narrow field of view of the SEM image presents a practical difficulty for the operator. Because SEM images are, by necessity, created within a vacuum chamber, direct observation of the specimen is not generally possible. Thus, the only visual feedback available to the operator is the SEM image itself. Since the operator can only “see” a small portion of the specimen at one time, it is thus difficult to “navigate” to a particular feature of interest or to move efficiently from one feature to another.
The term “navigation” is used advisedly in this context. Because of the relatively small field of view compared to the relatively large extent of the stage motion or the dimensions of the specimen(s), the SEM operator (particularly the novice) often feels very much like a mariner attempting to locate a landfall without charts or points of reference. Under these circumstances, it is not uncommon for the SEM operator to become confused and even move away from the desired feature. In some cases, operators have been known to damage specimens, staging mechanisms, or detectors due to inappropriate stage manipulation engendered by such confusion.
SEM specimens fall into two general categories: (1) single objects; and (2) collections of objects. As examples of the first category, a forensic specialist might wish to inspect a knife blade or an automotive engineer might wish to inspect a drive gear. In such cases, the portion of the object which can instantaneously be viewed in the SEM represents a fraction of its entirety and it can be quite difficult to locate a particular feature of interest. As an example of the second category, it is common to mount multiple small samples on a common “specimen carrier”. For instance, a very common mounting medium used for SEM is the “thumbtack” stub mount—a polished disk of typically one-half inch diameter with a peg protruding from the opposite side. In practice, small specimens (e.g., powders) are fixed to the polished surface and the stub is then mounted to a larger carrier plate which grips the mounting peg. In this manner, multiple stub-mounted samples are attached to a larger carrier which is then mounted on the SEM's positioning stage. The dimension of each stub is roughly comparable to the size of the SEM's maximum field of view, so it is relatively straightforward to navigate within its area, but it can be quite challenging to locate a particular stub on a carrier, particularly when the mounted specimens look similar. So in either case—the situation of single large objects or multiple smaller objects—the small field of view of the SEM creates a complication for efficiently locating a particular feature or object of interest.
Historically, the size of SEM samples and sample chambers has been steadily increasing. Many of the earliest SEMs could handle only one of the one-half inch diameter stubs described above. A modern SEM may be capable of handling very large specimens or very large arrays of specimens—objects of up to 13 inches in diameter or arrays of over one hundred individual specimens can be evaluated in some commercial units. Thus, the problem of efficient navigation has become increasingly important.
Since SEM images are produced by fundamentally different contrast mechanisms than are available in light microscopes, there are many practical situations where it is difficult to correlate a SEM image with an optical image of the same area. The lack of color information in a SEM image is a particularly important issue. Thus, visual “landmarks” may be lacking or obscured in a SEM image, further complicating the navigation problem.
Experienced SEM operators develop skills and techniques to confidently locate specimens and features even under the above-described circumstances, but such skills and techniques are, of course, not possessed by the novice or infrequent user. The SEM has increasingly moved from a role as a purely laboratory instrument into a role as an industrial tool. One consequence is that the individuals who are operating a SEM are less likely to be highly experienced “microscopists” and more likely to be technicians or engineers who are not highly experienced in operating the SEM. These latter individuals are less tolerant of learning specialized skills and more likely to commit errors of operation. Thus, the problem of locating features in a SEM is a practical problem of considerable consequence for the SEM manufacturer.
SEM manufacturers have long been aware of the above “navigation” issue and have devised various expedients to aid the user. The most basic of these expedients is the use of calibrated scales on the knobs used to manipulate the stage positioning controls. By becoming familiar with these scales, experienced microscopists are able to confidently manipulate the specimen to desired coordinates. As motorized stages have become more common and the use of computers to control them more prevalent, the basic mechanical scale concept has evolved correspondingly. Today, instead of turning knobs which directly move the stage, the operator may move the stage by means of a virtual or electronic “joystick” or similar device which communicates with motors which actually cause the stage to be translated. In such case, numerical readouts are commonly provided on the computer screen to indicate the position of the stage. Conceptually, however, this expedient is little more than a refinement of the position scale offered on the earliest SEM stages.
An important step up in sophistication is the provision of a graphical navigation “map”. This is simply a graphical representation of the area traversed by the stage and permits the operator to immediately visualize the position of the stage by means of a crosshair or other marker superimposed on the map. Such a “map” may, for example, appear as a “grid” whose equally spaced lines serve to provide relative indications of position. The operator is generally given the option of moving to a given point on the specimen by simply “clicking” a pointing device (such as a computer mouse) at the desired coordinates on the map. A further refinement is to provide a crude graphical representation of the specimen itself on the map (such as circles indicating the boundaries of standard sample stub positions). All of these refinements have been implemented in various forms and are much appreciated by SEM users. However, these are still relatively “abstract” aids and do not directly correspond to the operator's knowledge of the detailed visual morphology of the specimen. For example, a casual user is more likely to remember that the specimen of interest is a reddish-brown rectangle than that it is mounted on the second stub from the upper-right corner of the carrier.
Another distinctly different approach to the problem of navigating to a specimen feature is to offer the operator the means of actually seeing the specimen. One way of doing this is to provide a “viewport” which can be opened to peer into the vacuum chamber through a transparent window of some sort. Practical considerations of geometry and illumination limit the size and effectiveness of such provisions. Further, there are additional engineering considerations which make such viewports more difficult to accomplish than might first be thought—such as the need to provide an x-ray opaque viewing port and the need for an interlock mechanism to disable the light-sensitive imaging detectors when the viewport is opened. Consequently, most SEMs do not incorporate a viewport feature.
Another way of accomplishing much the same thing as the viewport is by interfacing a video camera to the vacuum chamber of the SEM. Some commercial SEMs have been produced which incorporate a video camera, together with an interlocked illumination system, mounted on a port such that the camera views the specimen under the beam. The video camera image provides a “live” view of the specimen, but since illumination must be provided to achieve this image, the normal imaging detectors of the SEM must be disabled to protect them from the effects of this illumination. Also, since the polepiece of the SEM's final probe-forming lens must be located directly over the specimen in order to image, this mandates that the camera view the specimen at a rather oblique angle. The polepiece itself is a rather large structure and the specimen must often be located within a few millimeters of it—this together with the fact that the region immediately above the specimen is rather crowded with other devices such as a backscattered electron detector, a secondary electron detector, and an x-ray detector, tends to restrict the field of view and thus compromise the practical utility of this arrangement as a navigation aid.
A variant of the video camera is the “chamber camera” which has become a moderately popular accessory for SEMs. This device is a small infra-red video camera which is equipped with an infra-red illumination source. Because SEM detectors are insensitive to weak IR illumination, this kind of camera can be used while the microscope is imaging. These devices produce monochrome images and, because they also suffer the same limitations of oblique and crowded viewing conditions as the aforementioned video cameras, they aren't particularly useful for locating features. Instead, their principal application is to help the operator manipulate irregular specimens safely. By mounting such a chamber camera roughly horizontal to the bottom of the polepiece, it is easy to see when the specimen is in danger of contacting the polepiece or one of the detectors. Though very useful, these devices do little to aid the practical problem of specimen navigation.
Electron microprobes (which can be considered to be highly specialized members of the SEM family) have long incorporated an optical microscope as part of their essential equipment. In the traditional design, the light path of the optical microscope is coaxial with the electron beam—a feat accomplished by specialized design of the electron optics. A less common variant is to provide a “side entry” microscope which views the specimen by means of a prism or mirror located immediately over the specimen. These devices, while useful for inspecting the feature being analyzed, have an inherently limited field of view (generally less than that of the electron optics in fact) and are thus of no practical value for specimen navigation.
One solution which has been described in the literature is to implement a separate microscope port with its optical axis parallel to, and offset from, the optical axis of the electron optics. This permits the specimen to be moved between the two viewing modalities by means of the stage. This expedient appears to have been implemented as a means of correlating SEM and light-microscope viewing modalities, rather than as a navigation aid, but it would also lend itself to use of a simple video camera which could capture a “macro” image. However, this implementation imposes both expense and complexity on the design of the microscope. Specifically, in order to allow both the SEM and the camera to view the entire specimen, the size of the specimen chamber and range of travel of the stage must be increased above the minimum required to do either separately. As a consequence, this implementation has not become widely used.
It should be noted that the several solutions described above which employ a camera device are relatively expensive to implement. On the one hand, this is due to the necessity of accommodating the camera to the SEM's vacuum system. Namely, either a vacuum-compatible viewing port must be implemented for the camera, or the camera must itself be vacuum compatible so that it can be contained in the chamber. Secondly, specialized illumination provisions must be implemented, which again increase the cost. This is in contrast to the present invention where there is no requirement for modifications to the specimen chamber, the use of a specialized or modified camera, or a specialized illumination provision.
Within the semiconductor industry, a concept superficially similar to some aspects of the present invention is employed for navigating semiconductor devices. This concept is to employ the design coordinates of a particular semiconductor feature to drive the SEM stage (or other inspection device) to the specified coordinates of the physical device for inspection. This is commonly implemented by means of a graphical user interface which may superficially resemble the kind of graphical user interface described in this invention. The distinction to be made is that the graphical user interface of the present invention is based upon an actual image of the physical device being examined, rather than a “virtual” image created by knowledge of the design parameters of the device, such as is the case for the common semiconductor “navigation” tool.
The invention which is the subject of this disclosure will be shown to be fundamentally different from all of the above implementations in respect to its method of implementation. It will also be demonstrated that an important part of the novelty of this invention is that it provides a very high degree of utility in addressing the fundamental SEM navigation problem with a notably simple and inexpensive apparatus.